1. Field of Invention
The present invention relates generally to bidirectional fiber optic communication systems and, more particularly, to an optical device which may be configured as an optical isolator and circulator for use in both high and low bit rate applications in such fiber optic systems.
2. Description of the Prior Art
Communication service providers are experiencing significant consumer demands to accommodate additional bandwidth in optically-based communications systems and the demand is ever-increasing. Today's optical communication systems and networks field rising consumer demands for e-mail, video, multimedia, data and voice-data transmission requirements across a variety of communication protocols. In the future, all indications are that the use of fiberoptic networks will become even more prevalent as a preferred medium for transferring information as the marketplace for wide-band services matures. It is anticipated that additional services such as enhanced pay-per-view, video-on-demand, interactive television and gaming, image networking, video telephony, CATV and ISDN switching services will be depend on and be substantial users of such systems. Because capacity is a critical parameter for system viability, bidirectional systems are desirable when the increased capacity or other attributes afforded by the bidirectional fiber is required. Enabling bidirectional use of installed and developing fiber in fiberoptic systems will permit communication service provider to gain additional utility from limited system resources.
Lasers are employed in numerous applications, particularly within fiberoptic communications networks, in which the laser emits an information-carrying light signal to an optical fiber which transmits the light signal to a photodetector for further processing. Typically, the optical signal propagates in one direction over a signal optical fiber. In a bidirectional fiber optic configuration, an optical signal propagates in both directions over an optical fiber. However, due to the sensitivities of these systems even a small amount of reflected light will cause instability to the laser source in terms of its power and frequency characteristics.
To reduce some of the problems of reflected light, optical circulators and isolators, non-reciprocal devices, may be installed at each end of a fiber link in a system thereby enabling the bit carrying capacity of an existing unidirectional fiber optic link. The use of Faraday isolators in optic systems is well known as an integral component for removing reciprocal light based on the use of polarizers which are rotated by 45 degrees relative to each other on either side of a magnetic medium, as optical isolators passes a signal in the forward direction from a first optical port to a second optical port. Optical circulators are employed in bidirectional systems for multiplexing the forward and reverse paths of an optical light source, such as a laser. Optical circulators provide a non-reciprocal coupling of light between two fiber paths, based on the Faraday rotation of light, as light is treated differently depending on its entry port into an optical port.
As is generally known, an optical circulator is a non-reciprocal optical device which allows the passage of light from a first port to a second port while a reverse optical signal into the second port is transmitted in totality to a third port; similar transmissions continue for remaining ports thusly creating a circulating operation. Effectively, any two consecutive ports of an optical circulator are an optical isolator as signals are transmitted only one way.
FIG. 1 is illustrative of the operation employing optical circulators to provide simultaneous, bidirectional communication in a single fiber optic link. In FIG. 1, optical circulators 100 and 200, each comprised of ports 10, 20, and 30, are installed at opposite ends of a fiber optic link 150. Communication transmitters 110 and 210 are connected to each port 10, communication receivers 120 and 220 are connected to each port 30, and the fiber optic link 150 is connected between ports 20 at each optical circulator. Light entering port 10 exits the optical circulator at port 20 as directionally indicated 160. Light that enters the optical circulator at port 20 exits at port 30 as directionally indicated 170. Light travels bidirectionally across a single fiber 150 as directionally indicated 180.
Using traditional beam splitting plates in bidirectional systems may result in substantial reductions in light intensity each time a light beam passes through a beam splitting plate (e.g. on the order of 50% loss). Typically these "ping-pong" type of fiber communications occur at low speeds. Additionally, there is an increase in noise to the system due to insertion loss, cross-talk and coupling loss. With the advent of long-haul applications, these reflective problems may cause the communication to fail; similarly, low speed rates are not well-suited for long-haul application. Wavelength dependent beam splitter cubes and dichroic mirrors are known in the field. U.S. Pat. No. 5,210,643 (Fuji et al.) discloses a wave combining apparatus having dichroic mirrors for combining laser beams having the same direction of oscillation by waveform division, respectively, and a polarizing beam splitting prism for combining the first and second resultant beams into a single combined waveform. However, the separation and recombination of optical beams as described in this reference is suited only for low-speed applications.
Therefore, the need exists for an apparatus which improves over the light intensity losses, virtually eliminates ghost images resulting from reflections, and is able to communicate in both high and low bit rate applications at a reasonably low-cost.